1. Field of the Invention
The present invention relates to a magnetic sensor for reading magnetically recorded information, a method of manufacture thereof and a magnetic storage incorporating the magnetic sensor.
2. Description of the Related Art
A magnetic sensor making use of magnetoresistance which changes an electric resistance in response to an external magnetic field, has been known as an excellent magnetic field sensor and already put to practical use as a reading element for detecting a signal magnetic field from a magnetic recording medium as a main component of a magnetic storage.
Magnetic storage technology is continuing to make significant improvements in their recording density. To achieve higher recording density, there are growing demands on magnetic heads for a narrow track width and higher read/write performances. As for the read characteristic, efforts are made to enhance a sensitivity of the head by advancing an MR head technology that utilizes the magnetoresistance. At a low a real density of several gigabits per square inch (Gb/in2), magnetic signals on recording media have been converted into electric signals by utilizing an anisotropic magnetoresistance (AMR). At higher a real densities a more sensitive giant magnetoresistance (GMR) is employed.
To meet the demand for even higher recording density, research and development are being conducted on a reading element which utilizes a GMR (CPP-GMR), the GMR that causes a sensing current to flow perpendicular to a layer plane, and a tunneling magnetoresistance (TMR). Both of these technologies are advantageous in increasing the sensitivity of the reading element as a distance between an upper shield layer and a lower shield layer (shield-to-shield distance) decreases.
When Barkhausen noise occurs, an S/N loss of the reading element increases and thus the Barkhausen noise needs to be minimized. The Barkhausen noise is produced by microscopic movements of domain walls and it is important to use a longitudinal bias layer that causes a free layer of a magnetoresistive multilayer film to have a single magnetic domain. This longitudinal bias layer is often arranged at both ends of the reading element in a track width direction when seen from the opposing recording media side.
The longitudinal bias layer is normally made from a hard magnetic material formed on an appropriate metallic underlayer. In the reading element described above in which a sensing current flows perpendicular to the layer plane, it is necessary to prevent a leakage of the sensing current as it flows through the longitudinal bias layer and also an output reduction caused by the leakage.
For example, JP-A-10-162327 discloses a structure in which an insulating layer is provided between the longitudinal bias layer and a magnetoresistive multilayer film, between the longitudinal bias layer and an upper shield layer, and between the longitudinal bias layer and a lower shield layer to prevent the leakage.
Further, JP-A-2001-6130 discloses a tunneling magnetoresistive head which has a nonmagnetic, conductive gap layer between the magnetoresistive multilayer film and the shields to eliminate a nonuniformity of the tunneling current.
As described above, the longitudinal bias layer is aimed at minimizing the Barkhausen noise by applying a magnetic field to a free layer of the magnetoresistive multilayer film to establish a single magnetic domain in the free layer. The lower shield layer and the upper shield layer are intended to absorb stray magnetic fields from neighboring bits or external fields so that information stored on the recording media can be read out correctly by the magnetoresistive multilayer film. Thus, the magnetoresistive multilayer film can detect a magnetic field of every bit recorded on the media.
When the head is actually manufactured, however, the distance between a longitudinal bias layer 1 and an upper shield layer 4 is smaller, as shown in FIG. 1, than that between the longitudinal bias layer 1 and a free layer 7 of the magnetoresistive multilayer film 2. This is attributed to the fact that, when a structure consisting of insulating layer 5/longitudinal bias layer 1/second insulating layer 6 is formed by the lift-off method at both ends of the magnetoresistive multilayer film 2, it is difficult for the second insulating layer 6 over the longitudinal bias layer 1 to have a sufficient thickness in the vicinity of the magnetoresistive multilayer film 2 because a width and an undercut length of a lift-off pattern are too small.
Because the longitudinal bias layer 1 is closer to the upper shield layer 4 than the magnetoresistive multilayer film 2, the amount of magnetic flux from the longitudinal bias layer 1 that is absorbed by the upper shield layer 4 is larger than that entering the free layer 7 of the magnetoresistive multilayer film 2. This in turn causes Barkhausen noise.